Entity storage

gistfile1.php

<?php

class DatabaseStorageController implements EntityStorageControllerInterface, ExtensibleStorageInterface {

   // Helper class implements the ExtensibleStorageInterface + more needed methods like write().
   protected $class = 'ExtensibleDBStorageHelper';

   protected function getExtensibleStorageHelper() {
      ...
   }


   function save() {
     // save base fields.
     $this->getExtensibleStorageHelper()->write(...);
   }

   /**
    * Implements EntityStorageControllerInterface - forward the calls to the helper.
    *
    */
   function createExtendedFieldStorage(..) {
     // equivalent of http://api.drupal.org/api/drupal/modules!field!field.api.php/function/hook_field_storage_create_field/7
     $this->getExtensibleStorageHelper()->createExtendedFieldStorage(..);
   }

}

/**
 * Converted field API storage
 */
class ExtensibleDBStorageHelper implements ExtensibleStorageInterface {


 // Has callbacks as field API storage, thus also implements write().



}


interface ExtensibleStorageInterface {

// Has only the callbacky of field API storage which do not involve entity CRUD, e.g. no write()

}


// Mongo can implement everything in one class if it wants to.
class MongoStorageController implements EntityStorageControllerInterface, ExtensibleStorageInterface {


   function save() {
     // Store the document in mongo.
   }

}